Ecohydrological interactions within “fairy circles” in the Namib Desert: Revisiting the self‐organization hypothesis

Abstract

Vegetation patterns such as rings, bands, and spots are recurrent characteristics of resource‐limited arid and semiarid ecosystems. One of the most recognizable vegetation patterns is the millions of circular patches, often referred to as “fairy circles,” within the arid grassland matrix extending over hundreds of kilometers in the Namib Desert. Several modeling studies have highlighted the role of plant‐soil interactions in the formation of these fairy circles. However, little is known about the spatial and temporal variabilities of hydrological processes inside a fairy circle. In particular, a detailed field assessment of hydrological and soil properties inside and outside the fairy circles is limited. We conducted extensive measurements of infiltration rate, soil moisture, grass biometric, and sediment grain‐size distribution from multiple circles and interspaces in the Namib Desert. Our results indicate that considerable heterogeneity in hydrological processes exists within the fairy circles, resulting from the presence of coarser particles in the inner bare soil areas, whereas concentration of fine soil occurs on the vegetated edges. The trapping of aeolian and water‐borne sediments by plants may result in the observed soil textural changes beneath the vegetation, which in turn, explains the heterogeneity in hydrological processes such as infiltration and runoff. Our investigation provides new insights and experimental data on the ecohydrological processes associated with fairy circles, from a less studied location devoid of sand termite activity within the circles. The results seem to provide support to the “self‐organization hypothesis” of fairy circle formation attributed to the antiphase spatial biomass‐water distributions.

Contrasting Global Patterns of Spatially Periodic Fairy Circles and Regular Insect Nests in Drylands

“…While there are multiple hypotheses for FCs origins, the two that enjoy current support are that dependent on faunal termite causation and on emergent vegetation patterning (Sahagian, ). Several studies have demonstrated that there are no direct observational data of past or present termite activities that support their potential role in the genesis of FCs in the central or southern Namib (Ravi et al, ; Tschinkel, , ) or in Australia (Getzin et al, ). There has also been no empirical verification of the role of either termites or vegetation patterning in the formation of FCs.…”

“…There has also been no empirical verification of the role of either termites or vegetation patterning in the formation of FCs. Although various termite species do occur in areas where FCs occur, their activities such as nests do not appear to correspond with regularity and spatial extent of FCs (Ravi et al, ). As a consequence, spatial pattern analysis of faunal features from other landscapes has been associated with the FC pattern (e.g., Tarnita et al, ).…”

The magic of fairy circles: Built or created?

“…The most famous examples are found in the Namib Desert. There has been an ongoing debate regarding the origin of these features, and a recent paper by Ravi et al (2017, doi:10.1002 sheds some light on this situation.The term stems from "Fairy Rings" in Europe, which are rings of mushrooms that are formed by a fungal mycelium growing outward from a central point. They are often found in forests but also appear in grassy meadows with disks of reduced grass growth surrounded by rings of thicker grass, due to the biogeochemical processes within the mycelium in a wet environment.…”

“…The most famous examples are found in the Namib Desert. There has been an ongoing debate regarding the origin of these features, and a recent paper by Ravi et al (2017, doi:10.1002 sheds some light on this situation.…”

“…In consideration of the Ravi et al [2017] paper, some difficult questions were addressed, while others remain. For example, the origin of "Fairy" circles is fairy rings of mushrooms in Europe, and these are clearly biologically controlled, by mycelium growing in a radial fashion, with rings growing over time.…”